Temperature compensator for fluxgate magnetometer

ABSTRACT

The present invention relates to a method and apparatus capable of being used with a fluxgate magnetometer or similar solenoid type device to compensate for temperature dependent variations in a reference magnetic field produced during operation of the device. A compensation solenoid is added and the effect of the temperature coefficient of resistivity of this compensation solenoid is used to compensate for the effect of the temperature coefficient of expansion of the reference field-producing solenoid.

United States Patent Ford Apr. 30, 1974 [54] TEMPERATURE COMPENSATOR FOR 2,848,656 8/1958 Nixon 317/ l3l G T GN M 3,051,873 8/ i962 Jensen 3l7/l3l lnventori James A. Ford, Silver Spring, Md.

The Johns Hopkins University, Baltimore, Md.

Filed: Dec. 14, 1972 Appl. No.: 315,014

Assignee:

US. Cl. 317/131, 324/43 R Int. Cl G01r 33/02 Field of Search 317/131; 324/43 R, 46,

References Cited UNITED STATES PATENTS 2/1944 Sias ..3l7/l3l 2/1950 Neild ..3l7/l3l Primary Examiner-L. T. Hix Attorney, Agent, or FirmRobert E. Archibald [57] ABSTRACT The present invention relates to a method and apparatus capable of being used with a fluxgate magnetometer or similar solenoid type device to compensate for temperature dependent variations in a reference mag netic field produced during operation of the device. A compensation solenoid is added and the effect of the temperature coefficient of resistivity of this compensation solenoid is used to compensate for the effect of the temperature coefficient of expansion of the reference field-producing solenoid.

7 Claims, 2 Drawing Figures ERROR SIGNAL '$S DETECTOR AMPLIFIER SIGNAL COIL l3 PATENTEDAPR301974 Y 1808508 SENSOR DRIVE SOURCE I gt ERROR SIGNAL TO AMPLIFIER l7 fi i IO SENSOR DRIVE DRIVE COIL H I SOURCE l5 MAGNETIC SIGNAL SGNAL DETECTOR ERROR SIGNAL l6 COMPENS 0N COIL NULLING TEMPERATURE COMPENSATOR FOR FLUXGATE MAGNETOMETER ACKNOWLEDGEMENT OF GOVERNMENTAL SUPPORT BACKGROUND OF THE INVENTION As is well-known, in modern fluxgate magnetometers the ambient magnetic field is nulled, at the sensor, by a magnetic field produced by the so-called nulling coil or solenoid; the accuracy of the instrument depending upon the value of the nulling current being proportional to the strength of the ambient magnetic field. This requires that the current applied to the nulling coil or solenoid must be constant fora constant ambient field. On the other hand, inasmuch as (a) the magnetic field generated by a solenoid is proportional to the turns per centimeter ratio for the solenoid, and (b) at various temperatures the expansion coefficient for the sensor materials may cause this turns ratio to vary slightly, it has been observed that the value of current required to be applied to the nulling solenoid often varies undesirably as a function of temperature. Obviously, such unwanted temperature dependency is not limited to fluxgate magnetometers, but can arise in various other devices in which an energizable coil or solenoid is employed to produce a reference magnetic field.

Several attempts have been made in the past to provide temperature compensation for such magnetometers or similar devices. However, for one reason or another these previously proposed compensation techniques have been found to be deficient; for example, due to a requirement for special solenoid or sensor materials, or the need for added amplifiers or other active components. In general these prior attempts at temperature compensation have significantly decreased the operating range and/or sensitivity of the device.

SUMMARY OF THE INVENTION In accordance with the present invention, temperature compensation is achieved by combining two similar solenoids to provide the effects of a single, temperature compensated solenoid. In the preferred embodiment of the invention, to be described in detail hereinafter, the effects of the temperature coefficient of resistivity of a compensating solenoid is used to compensate the effects of the temperature coefficient of expansion of the main or nulling solenoid of a fluxgate magnetometer. The proposed temperature compensation is accomplished without requiring amplifiers or other active components; no special materials are needed to match the temperature coefficients of the nulling and compensating solenoids; and, the proposed temperature compensator does not appreciably affect the operating range, sensitivity or time of the fluxgate magnetometer.

In view of the foregoing, one object of the present invention is to provide temperature compensation for devices of the type which utilize a solenoid to generate a reference magnetic field.

Another object of the present invention is to provide an improved temperature compensator for a fluxgate magnetometer.

A further object of the present invention is to provide a temperature compensator for a fluxgate magnetometer whereby the effect of the temperature coefficient of resistivity of a compensation solenoid is utilized to compensate the effect of the temperature coefficient of expansion of the magnetometer nulling solenoid.

' Other objects, purposes and characteristic features of the present invention will in part be pointed out as the description of the present invention progresses and in part be obvious from the accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration of a fluxgate magnetometer structure modified to provide temperature compensation in accordance with one embodiment of the present invention; and

FIG. 2 is a circuit diagram illustrating the circuitry employed in this fluxgate magnetometer embodiment of the proposed temperature compensating 'method and apparatus of the present invention.

Referring now to the drawings, the typical fluxgate magnetometer is enclosed by the heavy dashed line 9 in FIG. 2 and includes(see FIG. 1) a thin, substantially rectangular, magnetizable core 10 made of Mu metal or the like and having a plurality of coils or solenoids wound thereon. More specifically, a drive coil 11 is wound on one portion of the core 10 and is energized by a suitable source 12 of periodically varying drive signal. Typically, the frequency of this drive signal might be SKI-I2 for example. Wound at the opposite leg of the core 10 is a signal coil 13 which comprises a pair of oppositely wound sections, as shown, and which picks up a signal, from the core 10, which is rich in second harmonics of the drive frequency in the presence of a nonzero ambient magnetic field, represented by the arrow 14. The signal coil 13 is connected to a magnetic signal detector 15 of conventional design which responds to the signal picked up by coil 13 and produces a corresponding DC. output voltage level or error signal, designated at 16, proportional to the strength of the ambient field l4 and having a polarity indicative of the direction of field 14. The error signal 16 is applied as input to a suitable error amplifier 17 (see FIG. 2).

In accordance with conventional fluxgate magnetometer construction, a nulling coil 18 is wound around the core 10 overlying the drive and signal coils 11 and 13, and as is well-known to those skilled in this art, the coil 18 is energized with a current designated l, of sufficient magnitude to maintain the sensor in a nulled condition. The operation of a typical fluxgate magnetometer will be described in more detail hereinafter when discussing FIG. 2, suffice it to say here that the output of the sensor is a voltage signal proportional to the value of current 1 needed to null the sensor and therefore also proportional to the magnitude of the ambient magnetic field 14. In other words, the nulling coil 18 generates a magnetic field 19 which is intended to be exactly equal in magnitude but in opposite direction to the ambient field 14 being measured. Unfortunately, the magnetic field 19 generated by the coil 18 is proportional to its turns per centimeter ratio, and at various temperatures the expansion coefficient for the coil 18 causes this turns per unit length ratio to vary. As a result, the value of current I, required to be applied to the solenoid 18 in order to null the sensor, and therefore also the magnetometer output can be in error as functions of temperature; e.g., as temperature increases, the coil 18 would expand and anincreased amount of current I, would be needed'to null the sensor.

As noted earlier, in the illustrated embodiment of the present invention a compensation solenoid or coil designated at 20 is added to the conventional fluxgate magnetometer and, as will be described, the effects of the temperature coefficient of resistivity or resistance of the coil 20 are used to compensate the abovementioned adverse effects of the temperature coefficient of expansion of the nulling coil 18. Preferably the compensation coil 20 is wound tightly over the nulling coil 18 and is energized with a current designated I which is controlled to be just sufficient to cause the magnetic field 21 produced by the compensating coil 20, in the same direction as the ambient field l4 and opposite to the nulling field 19, to provide the desired compensation for the temperature variations ambient I magnetic core 10.

More specifically and in accordance with the present invention, a circuit network is added between the output of the error amplifier l7 and the nulling coil 18 and I 23-20. The output signal from the magnetometer is a voltage -developed across monitoring resistor. 24 and proportional to the value of current I, needed to null the magnetometer. The resistor 23 is selected to tailor the compensation coil 20 to the nulling coil 18;'i.e., a change in resistance at coil 20 with temperature is made equivalent to a change in turns per unit length ratio experienced by coil 18 due to this 'same temperature change. I

'By way of example, in one practical embodiment of the present invention, as applied to a fluxgate magnetometer, typical values of the various circuit components employed were as follows: resistors 22, 23 and 24 had resistance values of 51 ohms, l.2 kilohms and 2 kilohms respectively; whereas, both the nulling coil 18 and the compensation coil 20 were similar solenoids formed of N o. 30 wire and were approximately 0.5 inch in diameter, and each had a coil constant of 20 gammas per microamp. A typical temperature coefficient of expansionchange for nulling coil 18 might be 0. l 6 X percent of nominal length per degree centrigrade, based upon a nominal length measured at C for example, and a typical temperature coefficient of resistance change for the compensation coil might be approximately 0.4 percent of nominal resistance per degree Centigrade, based upon a nominal resistance also measured at 20C.

If, during operation of the temperature compensated magnetometer, the ambient temperature should increase, the resistance of the compensation solenoid 20 will also increase and thereby cause a corresponding decrease in the value of compensation current i and the opposing field 21 produced by coil 20, sufficient to properly compensate for the loss in turns per centimeter ratio which occurs at the nulling coil 18 due to this temperature increase. Conversely, when the ambient temperature decreases, the resistance of the compensation solenoid 20 will also decrease and thereby cause an increase in the compensation current and the strength of the opposing field 21, just sufficient to compensate for the decrease in the turns per centimeter ratio, at the nulling coil 18, which accompanies this same decrease in ambient temperature.

Only a small'percentage, e.g., approximately 3 percent, of thenulling currentl is used for the feedback or compensation current i so that, for practical designs, the change in the turns ratio of coil 20 will be negligible. Moreover, this minimal amount of feedback or compensation current 1;, does not significantly decrease the operating range or sensitivity of the magnetometer. For proper functioning of the proposed compensating method and apparatus ofthe present invention, compensating coil 19 must obviously be located at the sensor, as previously noted; whereas, the resistances 22 and 23 may be located in the fluxgate electronic package. It should also be noted here that the limited to use with a fluxgate magnetometer, but might also find application with similar devices such as, for example, a Helmholtz coil pair.

Various other modifications, adaptations and alterations are of course possible in light of the above teachings. It should therefore be understood at this time that within the scope of the appended claims the invention may be practiced otherwise than as specifically described hereinabove.

What is claimed is:

1. A coil device having temperature compensation circuit means and comprising,

a first solenoid for producing when energized with a controlled value of electric current a reference magnetic field, 1

said first solenoid being characterized by a turns per unit length ratio which varies with changing ambient temperature, and 1 I a compensation circuit means including a second solenoid connected to be energized with a predetermined portion of said controlled value of electric current,

said second solenoid being characterized by a resistivity which varies with changing ambient temperature and producing when energized a magnetic field which when summed with said reference field results in a constant magnetic field for a given value of said controlled energizing current notwithstanding any change in ambient temperature. I 2. The coil device specified in claim 1 wherein the turns per unit length ratio of said first solenoid decreases with increasing ambient temperature, and wherein the resistivity of said second solenoid increases with increasing ambient temperature.

3. The coil device specified in claim 2 wherein said first solenoid is connected in a series energizing circuit including a resistance means and a source of controlled energizing current, and said second solenoid'is connected in a second energizing circuit electrically in multiple with said resistance means.

4. The coil device specified in claim 3 wherein said second energizing circuit means further includes a second resistance means connected in series with said second solenoid and having a resistance value effective to tailor said second solenoid to said first solenoid.

5. The coil device specified in claim 1 and further including,

a magnetizable core having said first and second solenoids wound thereon,

said first solenoid possessing a turns per unit length ratio which varies inversely and said second solenoid possessing a resistivity which varies in direct proportion respectively with the temperature ambient said core,

drive means having a drive coil means wound on said core for creating within said core a periodically varying signal,

signal coil means wound on said core for picking up a signal indicating the presence of a non-zero magnetic field ambient said core, and

circuit means responsive to the signal picked up by said signal coil means for energizing said first solenoid with a controlled current value effective to cause the reference magnetic field produced by said first solenoid to be directed opposite from and to null said ambient magnetic field, whereby the value of said energizing current to said first solenoid needed to null said ambient magnetic field produces an output signal indicating the strength of said ambient magnetic field.

6. The coil device specified in claim 5 wherein,

said energizing circuit means for said first solenoid includes a first resistance means connected in series with said first solenoid, and

said second solenoid is connected in circuit multiple with said first resistance means to be energized by a preselected portion of the energizing current to said first solenoid,

said second solenoid having a winding sense opposite to that of said first solenoid whereby said second solenoid produces when energized a magnetic field which opposes the magnetic field produced by said first solenoid.

7. The coil device specified in claim 6 further including,

a second resistance means connected in series with said second solenoid and in multiple with said first resistance means for equating a change in the resistance of said second solenoid for a given change in temperature to the change in turns per unit length experienced by said first solenoid as a result of the same temperature change. 

1. A coil device having temperature compensation circuit means and comprising, a first solenoid for producing when energized with a controlled value of electric current a reference magnetic field, said first solenoid being characterized by a turns per unit length ratio which varies with changing ambient temperature, and a compensation circuit means including a second solenoid connected to be energized with a predetermined portion of said controlled value of electric current, said second solenoid being characterized by a resistivity which varies with changing ambient temperature and producing when energized a magnetic field which when summed with said reference field results in a constant magnetic field for a given value of said controlled energizing current notwithstanding any change in ambient temperature.
 2. The coil device specified in claim 1 wherein the turns per unit length ratio of said first solenoid decreases with increasing ambient temperature, and wherein the resistivity of said second solenoid increases with increasing ambient temperature.
 3. The coil device specified in claim 2 wherein said first solenoid is connected in a series energizing circuit including a resistance means and a source of controlled energizing current, and said second solenoid is connected in a second energizing circuit electrically in multiple with said resistance means.
 4. The coil device specified in claim 3 wherein said second energizing circuit means further includes a second resistance means connected in series with said second solenoid and having a resistance value effective to tailor said second solenoid to said first solenoid.
 5. The coil device specified in claim 1 and further including, a magnetizable core having said first and second solenoids wound thereon, said first solenoid possessing a turns per unit length ratio which varies inversely and said second solenoid possessing a resistivity which varies in direct proportion respectively with the temperature ambient said core, drive means having a drive coil means wound on said core for creating within said core a periodically varying signal, signal coil means wound on said core for picking up a signal indicating the presence of a non-zero magnetic field ambient said core, and circuit means responsive to the signal picked up by said signal coil means for energizing said first solenoid with a controlled current value effective to cause the reference magnetic field produced by said first solenoid to be directed opposite from and to null said ambient magnetic field, whereby the value of said energizing current to said first solenoid needed to null said ambient magnetic field produces an output signal indicating the strength of said ambient magnetic field.
 6. The coil device specified in claim 5 wherein, said energizing circuit means for said first solenoid includes a first resistance means connected in series with said first solenoid, and said second solenoid is connected in circuit multiple with said first resistance means to be energized by a preselected portion of the energizing current to said first solenoid, said second solenoid having a winding sense opposite to that of said first solenoid whereby said second solenoid produces when energized a magnetic field which opposes the magnetic field produced by said first solenoid.
 7. The coil device specified in claim 6 further including, a second resistance means connected in series with said second solenoid and in multiple with said first resistance means for equating a change in the resistance of said second solenoid for a given change in temperature to the cHange in turns per unit length experienced by said first solenoid as a result of the same temperature change. 